'^t 



Figure 365.— Initial stage in the formation of an oyster bank on very soft mud of a tidal flat. Photographed at low 



tide near Brunswick, Ga. 



WATER MOVEMENTS 



Free exchange of water is essential for the 

 growth, fattening, and reproduction of oysters. 

 An ideal condition is represented by a steady, 

 nonturbulent flow of water over an oyster bed, 

 strong enough to carry away the liquid and gaseous 

 metabohtes and feces and to provide oxygen and 

 food. Furthermore, an oyster bed can expand 

 only if the larvae are carried by the currents and 

 at the time of setting are brought in contact 

 with clean, hard sm-faces. Estuaries seem par- 

 ticularly suitable for the expansion of oyster 

 communities and for the annual rehabihtation of 

 oyster populations reduced by harvesting because 

 some larvae, carried back and forth by the oscil- 

 lating movements of tidal waters, eventually 

 settle beyond the place of their origin. 



In large embayments, such as Long Island 

 Sound, the difference between the surface and 

 bottom sahnities is small, about l°/oo or 2%©. 

 In tidal rivers and true estuaries the differences 

 between the salinities of the lower strata and those 

 at the surface are considerable. Salinity strati- 



fication, as will be shown later, complicates the 

 pattern of circulation. 



The great variety of conditions found in the 

 bodies of water within the tidal zone nuikes it 

 difficult to define the term "estuary" in a few 

 precise words. A Latin dictionary (Andrews, 

 1907) defines the word "Aestuarium" or "Aestus" 

 as a part of the seacoast overflowed at flood tide 

 Init at ebb tide left covered with mud and slime. 

 Some authors extend the concept of an estuary to 

 include such large bodies of water as the Mediter- 

 ranean Sea and the Gulf of Mexico, while others 

 restrict the use of the term to relatively small 

 coastal indentures in which the hydrogi-aphic 

 regime is influenced by the river discharge at the 

 head and the intrusion of sea water at the mouth. 

 Cameron and Pritchard (1963) define an estuary 

 as "a semienclosed coastal body of water having 

 free connection with the open sea and within 

 which the sea water is measurably diluted with 

 fresh water deriving from land drainage." The 

 essential features of a true estuary are the inflow 

 of river water at the head and the periodical 

 intrusion of sea water at its mouth. Stommel 

 (1951) classifies estuaries by the predominant 



400 



FISH AND WILDLIFE SERVICE 



